1. The Field of the Invention
The present invention relates to the field of bandgap voltage reference circuits. In particular, the present invention relates to circuits and methods for providing a bandgap voltage reference less dependent on or independent of a resistor ratio.
2. The Prior State of the Art
The accuracy of circuits often depends on access to a stable bandgap voltage reference. Accordingly, numerous bandgap voltage reference circuits have been developed. Bandgap voltage reference circuits will also be referred to herein as xe2x80x9cbandgap references.xe2x80x9d A traditional bandgap reference generates a bandgap voltage reference that is stable with temperature by summing a relatively small Proportional To Absolute Temperature (PTAT) voltage (VPTAT) with a base-emitter voltage (VBE) of a bipolar transistor to generate a bandgap reference voltage that is stable with temperature.
FIG. 1 schematically illustrates a conventional bandgap reference 100 in accordance with the prior art. The bandgap reference 100 includes a PTAT voltage generator 101 that generates the PTAT voltage VPTAT. The PTAT voltage generator 101 is coupled to a bipolar transistor, which is in turn coupled to a current bias 103 as illustrated. The result is an output voltage VOUT that is equal to the sum of VPTAT and VBE. The positive temperature drift of VPTAT largely compensates for the negative temperature drift of VBE thus resulting in the output voltage VOUT being relatively stable with temperature.
FIG. 2 illustrates a conventional PTAT voltage generator 200, which may be the PTAT voltage generator 101 of FIG. 1. The PTAT voltage generator 200 includes four equivalently-sized bipolar transistors 201 through 204 coupled together as shown, and having an emitter terminal coupled to a corresponding current source 211 through 214. The current sources 211 and 212 are xe2x80x9cMxe2x80x9d times the magnitude of the current sources 213 and 214. The emitter terminals of the bipolar transistors 202 and 203 are each coupled to an input of an operational amplifier 224. The output of the amplifier 224 is coupled to ground via a series of elements that includes a resistor 222 having a resistance R2, a resistor 221 having a resistance R1, and a bipolar transistor 223, as shown.
In the illustrated configuration, the voltage across the resistor 221, which will be referred to as V1, is defined by the following Equation (1).
V1=2UTln(M)xe2x80x83xe2x80x83(1)
where,
M is equal to the current ratio between current sources 211 and 212 and current sources 213 and 214; and
UT is often referred to as the xe2x80x9cthermal voltagexe2x80x9d and is equal to       kT    q    .
Note that k is Boltzmann""s constant (1.38xc3x9710xe2x88x9223 Joules(J)/Kelvin(K) or 8.62xc3x9710xe2x88x925 electron volts (eV)/K), T is temperature in degrees Kelvin, and q is the magnitude of charge of an electron (1.60xc3x9710xe2x88x9219 Coulombs(C)). In addition, the voltage across both,resistors 221 and 222, which will be referred to as VPTAT, is defined by the following Equation (2).                               V          PTAT                =                              (                          1              +                                                R                  1                                                  R                  2                                                      )                    ⁢          2          ⁢                      U            T                    ⁢                      ln            ⁡                          (              M              )                                                          (        2        )            
In order to compensate for the negative temperature drift of the bipolar transistor 102, the PTAT voltage generator 101 needs a PTAT voltage VPTAT of approximately 33ln(2)UT. The resistor ratio R1/R2 of the PTAT voltage generator 200 may thus be adjusted so that the PTAT voltage VPTAT approximates 33ln(2)UT. In the case of the design in FIG. 2 with the density ratio M being 100, the resistor ratio R1/R2 would be approximately 1.48. Although there are a variety of circuits for providing a PTAT voltage, such circuits typically employ a resistor ratio in order to provide the needed level of positive temperature shift.
Resistors can often take up significant chip space. With integrated circuits becoming increasing compact and complex, there is an effort to reduce the size of circuitry where possible. Accordingly, what is desired are circuits and methods for providing a bandgap voltage reference in a more compact fashion.
The foregoing problems in the prior state of the art have been successfully overcome by the present invention, which is directed to circuits for providing a bandgap voltage reference that is less dependent on a resistor ratio. By reducing the dependency on the resistor ratio, the resistor ratio may be lowered thereby reducing the size of the resistors that generate the resistor ratio. In one embodiment, the dependency on a resistor ratio is eliminated completely, in which case there is not need for a resistor ratio at all.
Conventional bandgap voltage references use a single Proportional To Absolute Temperate (PTAT) source to generate a small PTAT voltage. That voltage is then added to a base-emitter voltage of a bipolar transistor to generate an accurate bandgap voltage. Conventional PTAT sources typically use a resistor ratio to generate the PTAT voltage. However, contrary to conventional technology, the principles of the present invention use more than one PTAT source coupled in series. The PTAT voltage generated by all previous PTAT sources in the series are added to the supplemental PTAT voltage generated by the next PTAT source in the series, and so forth, until the final PTAT voltage has been generated by the terminating PTAT source in the series.
One might think that the addition of supplemental PTAT sources might increase the size of the overall bandgap generation circuit. However, in many applications, the bandgap voltage references in accordance with the present invention may be made smaller when factoring in that the resistor ratio dependency is reduced or even eliminated.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.